1. Field of the Invention
This invention relates generally to moisture-curable compositions and, in particular, to those having reactive silane functionality. The compositions of the invention may be cured on demand using an acid generating material. The invention further relates to methods of using such compositions.
2. Description of the Related Art
Moisture-curable compositions cure in the presence of moisture to form crosslinked materials such as adhesives, sealants and coatings that are useful in many industries. The moisture is typically obtained from the atmosphere or from a substrate to which the composition has been applied, although it may be added to the composition.
Moisture-curable compositions usually comprise polymers having groups (for example, alkoxysilane or acyloxysilane moieties) that react in the presence of moisture to form cured (i.e., crosslinked) materials. A wide variety of polymers may be rendered moisture curable, including polyolefins, polyesters, polyethers, polyacrylates, polyvinyl chloride, polyphosphazenes, polysiloxanes, polysulfides, block copolymers and fluorinated derivatives thereof, the particular polymer being selected based on the intended use. For example, a polysiloxane or fluorinated polyether is often selected to provide release coatings suitable for use with pressure-sensitive adhesives.
Moisture-curable compositions comprising alkoxysilane or acyloxysilane functionality typically cure in two reactions. In the first reaction, the alkoxysilane or acyloxysilane groups hydrolyze in the presence of moisture and a catalyst to form compounds having silanol groups. In the second reaction, the silanol groups condense with other silanol, alkoxysilane, or acyloxysilane groups in the presence of a catalyst to form --Si--O--Si-- linkages. The two reactions occur essentially simultaneously upon generation of the silanol-functional compound. Commonly used catalysts for the two reactions include Bronsted and Lewis acids and are described in the Encyclopedia of Polymer Science and Engineering, 2nd Edition, Volume 15, page 252, (1989). A single material may catalyze both reactions.
The hydrolysis reaction is schematically illustrated below for a polymer having alkoxysilane groups: ##STR1##
The resulting silanol (SiOH) groups are not stable in the presence of the acid catalyst and immediately condense with other silanol or alkoxysilane group to form --Si--O--Si-- linkages as shown below schematically for the condensation reaction of a silanol-functional compound and an alkoxysilane-functional compound: ##STR2##
Preferably, the hydrolysis and condensation reactions proceed quickly once the moisture-curable composition has been applied, for example, to a substrate. At the same time, however, the reactions must not occur prematurely, for example, during processing or storage.
A good balance between these properties is often difficult to obtain as rapid reactivity and storage stability are opposite properties to each other. For example, highly active catalysts such as tetraalkyl titanate esters rapidly accelerate the moisture-curing reaction but at the same time make it difficult to process the materials without risking premature gelation in feed tanks, coating equipment, and other manufacturing and handling apparatus. On the other hand, conventional catalysts such as dibutyl tin dilaurate yield stable moisture-curable compositions that may be processed and stored without premature gelation, but the cure rate is often too slow to be commercially practical for most manufacturing operations.
A variety of approaches have been used for providing moisture-curable compositions that have acceptable cure rates without processing and storage difficulties. For example, U.S. Pat. No. 2,843,555 describes a two-part system, one part comprising a functional polymer and the other part comprising a catalyst with the two parts being mixed just prior before use. While this approach is useful in small-scale applications, it is less efficient for large-scale manufacturing where delays caused by having to mix the two parts are undesirable. Furthermore, coating operations must be completed expeditiously before the composition cures in the pot, and this can be difficult when working with large surface area substrates or a large volume of composition.
U.S. Pat. No. 5,286,815 discloses an ammonium salt catalyst that is inactive until heated sufficiently to liberate an acid compound that initiates the moisture curing reaction. However, liberation of the acid also generates an amine that must be removed by evaporation. In addition, the heat used to activate the catalyst may damage heat-sensitive substrates onto which the composition has been applied.
An article entitled "Cationic Photopolymerization of Ambifunctional Monomers" (J. V. Crivello et al., Macromolekular Symposia, 95, 79-89, (1995)) describes the photopolymerization of "Ambifunctional" monomers (i.e., monomers bearing two chemically different reactive functional groups within the same molecule) using cationic catalysts. In one example, an ambifunctional monomer having both epoxycyclohexyl and trimethoxysilyl reactive functional groups is prepared and then subsequently UV irradiated in the presence of a cationic triarylsulfonium catalyst. According to the authors, the polymerization of these monomers, " . . . is characterized by a rapid polymerization of the epoxy group followed by a slow consumption of the trialkoxysilyl groups."
Thus, there remains a need for moisture curable compositions that afford acceptable cure rates without processing and storage difficulties (e.g., premature gelation). Ideally, these compositions will be able to be processed efficiently, for example, without the need for mixing prior to cure, and will employ catalysts that do not generate species that have to be removed. Preferably, the compositions will not require heat activation so as to avoid damage to a heat sensitive substrate.